A prior art multi perspective scanning microscope (MPSI) system 10 is described in FIG. 1. System 10 includes an electron gun (not shown) for generating a primary electron beam, as well as multiple control and voltage supply units (not shown), an objective lens 12, in-lens detector 14 and external detectors 16. System 10 also includes deflection coils and a processor (not shown).
In system 10 the primary electron beam is directed through an aperture 18 within the in-lens detector 14 to be focused by the objective lens 12 onto an inspected wafer 20. The primary electron beam interacts with wafer 20 and as a result various types of electrons, such as secondary electrons, back-scattered electrons, Auger electrons and X-ray quanta are reflected or scattered. Secondary electrons can be collected easily and most SEMs mainly detect these secondary electrons.
System 10 is capable of detecting some of the emitted secondary electrons by in-lens detector 14 and by external detectors 16.
Objective lens 12 includes an electrostatic lens and a magnetic lens that introduce an electrostatic field and a magnetic field that leak from the lens towards the wafer. The collection of secondary electrons is highly responsive to the leaked electrostatic field while it hardly influenced by the leaked magnetic field.
The leaked electrostatic field attracts low energy secondary electrons and very low energy secondary electrons into the column. A significant part of the very low energy secondary electrons are directed through the aperture of in-lens detector 14 and are not detected. Low energy secondary electrons are directed towards the in-lens detector 14. High-energy secondary electrons are detected if their initial trajectory is aimed towards one of the detectors. Very low energy is typically below 2 eV, while low energy typically ranges between 2 eV and 15 eV.
In practice, when the working distance (between the column lower end and the wafer) is decreased (for example, below 0.5 mm), or when the cap voltage (the voltage that is applied to a lower portion of the electrostatic lens) is increased (for example, above 1 kV), most of the secondary electrons are not detected at all. They will enter the aperture of the In-lens detector 14. Such a decrement (in working distance) and/or increment (in voltage cap) also direct less secondary electrons to be directed towards the external detector, and vise versa.
Effective defect review tool requires both types of detectors in order to capture all types of defects. In-lens detector 14 is usually used for determining a contrast between different materials, and is also useful in voltage contract mode as well as in HAR mode. The In-lens detector 14 is also very sensitive to pattern edges. External detectors 16 are much more sensitive to the topography of the wafer. They external detectors are also less susceptible to wafer charging, which is significant when imaging highly resistive layers.
The working distance and the cap voltage also influence the resolution of the system. A decrease in the working distances reduces the chromatic aberration thus improving resolution, and vice verse.
As illustrated above if the working distance is decreased the resolution improves but the amount of detected electrons decrease.
There is a need to provide a system and method that allows both high-resolution and multi perspective capabilities.
U.S Pat. No. 6,555,819 of Suzuki et al (which is incorporated herein by reference) describes a multi-detector SEM having magnetic leakage type objective lens where the magnetic field largely influences the trajectory of emitted secondary electrons. This SEM has various disadvantages, such as not being capable of providing tilted images. Suzuki has a reflector that includes an aperture through which the primary electron beam passes, thus reflected electrons may pass through this aperture and remain un-detected.